Head-up displays and sights available on the market today present a large size and an expensive price tag. This makes them unsuitable for small aeroplanes and helicopters. The sights are normally also provided with some angular magnification and a corresponding smaller field of view, which limits their use to aiming of weapons.
U.S. Pat. No. 4,697,879 disclose an airborne display system for viewing high-definition collimated images. The system includes a high-key image generator, a head-down collimating assembly, an optical relay device, and an optical mixing device. The system is devised to display images from two image sources.
U.S. Pat. No. 5,943,171 disclose a head mounted display for entering an image into the eye of a viewer along a viewing axis. The display includes polarizers and a concave mirror.
U.S. Pat. No. 5,596,451 disclose an assembly for producing a visual display including a light-reflecting type spatial light modulator and illumination arrangement for providing a particular source of light.
WO 95/10106 disclose a binocular head mounted display unit.
There is a need for low-price, non-bulky head-up displays suitable for use in small vehicles such as small aeroplanes and helicopters.
One object of the present invention is therefore to provide a head-up display system that is compact in size. It is also a further objective of the present invention to provide a head-up display system that comes with a low cost.
The image that is to be shown to the pilot may comprise symbols and aiming marks, generated by an accompanying computer and electronics. Further objectives include that the direction accuracy and stability of these symbols and marks must be good enough for views for sighting. An image from a thermal imaging device should also be possible to superimpose on the pilot's view of the terrain and/or the runway issuing the angular magnification of 1, to make it possible to achieve a safe landing even during bad weather and sight conditions. Another object of the present invention is therefore to provide equipment with an image resolution corresponding to SXGA or better (1400×1050).
The image that is to be shown for the pilot have to be collimated, (appear to be at a large distance), which implies that the pilot can move his head without affecting the direction (free of parallax), and there is no need for accommodation of the pilots eyes as he shift his view from the terrain to the symbols/thermal image.